Euro-format refills are a bit of an oddity - most 'standard' refills are quite a well-defined shape and size, and although there can be the odd exceptions, are usually all interchangeable. Euro-format rollerball refills aren't quite as standard as you might like. Although most pens that use these refills can use any of them, they're not universal. Some have a little step at the 'shoulder', which others don't have. The bit just behind the tip can vary in width, so some refills don't go in some pens. Some are a little bit thinner at the tip, so other refills don't fit that brand's pens.
It's all a bit of a pain, so it does mean you're usually safest sticking with the pen manufacturer's own refills, but if you don't mind taking a bit of a risk, there are lots of options here that will probably fit. The refills listed here should stick to the 'standard' shape fairly well.
If there isn't a refill specific to your pen's brand, the best bet is the Schmidt 888 or 5888. These are the most common refills supplied with pens from all sorts of other brands, and a few other refills from well-known brands are just these with different printing on them!
Along with the common liquid-ink rollerball refills here, there are a few fineliner refills, which give you a quite different choice for writing and drawing.